Method and apparatus for increasing writing rate of storage tube



Nov. 26, 1968 J, J, DONOGHUE ET AL 3,413,513

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BUG/(HORN, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNE)? I Nov. 26, 1968 J. J.DONOGHUE ETAL 3,413,513 METHOD AND APPARATUS FOR INCREASING WRITING RATEOF STORAGE TUBE Original Filed Jan. 13, 1964 2 Sheets-Sheet 2 G E 0VTIME o v ar 1 5 6 JAMES .1. DONOGHUE mam/#0 a. McM/LLAN, .1. BINVENTORS.

BUG/(HORN, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS United States Patent3,413,513 METHOD AND APPARATUS FOR INCREASING WRITING RATE OF STORAGETUBE James J. Donoghue, Portland, and Richard B. McMillan,

Jr., Tigard, Oreg., assignors to Tektronix, Inc., Beaverton, 0rcg., acorporation of Oregon Continuation of application Ser. No. 337,370, Jan.13, 1964. This application Apr. 7, 1967, Ser. No. 659,826

14 Claims. (Cl. 315-11) ABSTRACT OF THE DISCLOSURE A bistable storagetube and a method of operating the same to increase its writing rate. Anenhancement pulse is applied to the flood gun cathode which is muchgreater than the first crossover voltage to enable storage of writtencharge images of initially low voltage. The pulse is terminated at suchtime as to prevent the potential of the unwritten background areas ofthe storage dielectric from increasing above the first crossovervoltage. This is possible because the more positive voltage portions ofthe storage dielectric charge at a faster rate than the more negativeunwritten background target areas.

The present application is a continuation of copending US. patentapplication Ser. No. 337,370, filed Jan. 13, 1964, now abandoned, byJames J. Donoghue et al.

The subject matter of the present invention relates generally toelectron image storage apparatus, and in particular to methods andapparatus for operating a bistable storage tube to increase its Writingrate so that the electron beam within such tube may be moved across itsstorage target at a faster rate and still produce a charge image whichis stored an indefinite controlable time.

Briefly, one embodiment of the method of operation of the presentinvention involves turning off the flood electron guns within such tubeto prevent low velocity flood electrons produced by such flood gun fromstriking the storage target during the time such target is bombarded bya beam of high velocity writing electrons to produce the charge image onsuch target. Then the target is bombarded with flood electrons ofincreased velocity immediately after the charge image is formed by thewriting electrons after which the velocity of such flood electrons isdecreased below that which will drive the potential of the backgroundareas of the storage target above the first cross over voltage of thesecondary emission characteristic of such target at which the secondaryemission ratio is one. As a result of the momentary increase in velocityof the flood electrons, the potential of any portion of the charge imagewhich is initially below the first cross over voltage, is driven abovethe first cross over voltage so that the entire charge image can bestored as a bistable image for an indefinite controlable time. The firstcross over voltage is the minimum voltage difference between thebombarded rear surface of the storage dielectric of the target and theflood gun cathode which is necessary to enable bistable storage.

The present method of operating the storage target to increase itswriting rate is especially useful when employed for a cathode rayoscilloscope having a direct viewing bistable storage tube as its signaldisplay device. This enables high frequency input signals or thosehaving extremely fast rise times to be stored on the target of thestorage tube of such oscilloscope even though such signals are above thenormal writing rate of such target achieved by conventional operation.In addition, the method of the present invention may be employed forstoring both repetitive signals and transient signals. However highfrequency repetitive signals may be stored by charge image inte-3,413,513 Patented Nov. 26, 1968 gration merely by preventing the floodelectrons from striking the storage target during the bombardment ofsuch target by the writing beam for several consecutive cycles of suchrepetitive input signal so that charge images of their successivewaveforms are superimposed to increase the potential of the waveformimage formed on the target above the first cross over voltage. Thischarge image integration is more fully discussed in copending US. patentapplication Ser. No. 302,880, filed by Robert H. Anderson on Aug. 19,1963, and entitled, Improved Storage Tube and Method of Operation, nowPatent No. 3,325,673. The present method may be used in place of chargeimage integration when there is jitter in the repetitive signal so thatsuccessive waveforms are not superimposed on the storage target, or itmay be combined with charge image integration to enable faster storage.Since charge image integration is obviously not possible with atransient signal the method of the present invention is even more usefulwhen applied to enable the storage of transient signals having fast risetimes which would not otherwise be stored by conventional operatingtechniques.

The method of operation of the present invention has several advantagesover conventional methods of operating storage tubes includingincreasing the frequency response of the storage tube by effectivelyincreasing th writing rate of the storage target of such tube in asimple and efficient :manner. The present method may be applied to allconventional storage tubes including direct readout tubes employing agrid control or transmission type storage target and a separate phosphorscreen spaced from such target, as Well as nonconventional tubesemploying phosphor storage targets such as are described in copendingUS. patent applications Ser. No. 180,457, filed Mar. 19, 1962, by RobertH. Anderson, and entitled, Electron Discharge Display Device, now PatentNo. 3,293,473, and Ser. No. 299,422, filed Aug. 1, 1963, by Charles B.Gibson, entitled, Storage Target for Cathode Ray Tube and PhotographicMethod of Manufacture, now Patent No. 3,293,474.

Apparatus for carrying out the method of operation of the presentinvention is simple and inexpensive. In order to provide manualoperation, one embodiment of this apparatus is in the form of a manualswitch having a movable contact connected to the cathodes of the floodguns within the storage tube with one of its fixed contacts connected toa source of positive D.C. voltage source above cut off and another ofits fixed contacts connected to the common terminal of a capacitor and acharging resistance which are connected in series between a negative DC.voltage source and ground. Alternatively, the apparatus may be of a typeto provide automatic operation, in which case it may include a blankingsignal generator which is triggered by the input signal applied to thevertical deflection plates of the storage tube, and applies a negativevoltage blanking pulse to the control grids of the flood guns to cut offsuch flood guns so that a capacitor connected to the cathode of eachflood gun charges to a suitable negative DC voltage to increase thevelocity of the flood electrons when such flood gun is turned back on bytermination of the blanking pulse. This automatic or triggered operationhas the advantage that it enables the writing rate enhancement pulse tobe applied immediately after the input signal has written its chargeimage on the storage target before the potential of such charge imagedecreases appreciably due to leakage. Therefore, such triggeredoperation is very desirable when storing high speed transient signalswhose charge images are initially of very low potential.

It has also been discovered that target areas of different initialvoltage potential charge at different rates when low velocity floodelectrons bombard such target areas. A target area of higher potentialcharges at a faster rate than a target area of lower potential so thatthe potential difference between such target areas increases with timeduring charging. This means that if the waveform charge image producedon the storage target is of only a slightly greater potential than thebackground areas of the storage targets, it is still possible to storesuch charge image bistably by increasing the potential of the chargeimage and the background areas by applying a negative enhancement pulseto the flood gun cathode until the potential of the charge image isgreater than the first cross over voltage but terminating theenhancement pulse before the potential of the background areas exceedthe first cross over voltage. It is found that this increased potentialdifference between written target areas and unwritten target areas dueto their different charging rates, may be further increased by employinga spike enhancement pulse having an amplitude greater than the firstcross over voltage and exponential trailing edge which can be producedby discharging a suitable capacitor through resistance of the propervalue.

It is therefore one object of the present invention to provide animproved method and apparatus for operating a storage tube to increasethe writing rate of such storage tube.

Another object of the present invention is to provide an improved methodand apparatus for operating a bistable storage tube which allows suchtube to store higher speed transient signals as well as repetitivesignals of greater frequency.

A further object of the invention is to provide a simple method ofincreasing the writing rate of a bistable storage tube by preventing lowvelocity holding electrons from striking the storage target during thetime the charge image is produced on such target by high velocitywriting electrons and momentarily increasing the velocity of the holdingelectrons striking the storage target immediately after the charge imageis produced.

An additional object of the present invention is to provide a simple andinexpensive electrical circuit which may be operated by a manual switchfor increasing the writing rate of any bistable storage tube.

A still further object of the present invention is to provide anelectrical circuit for increasing the writing rate of a bistable storagetube automatically by triggering such circuit in response to the inputsignal to be stored.

Additional objects and advantages of the present invention will beapparent from the following detailed description of certain preferredembodiments thereof and from the attached drawings of which:

FIG. 1 is a schematic diagram of one embodiment of the apparatus made inaccordance with the present invention;

FIG. 2 is a curve of the voltage applied to the cathode of the floodguns employed in the storage tube of FIG. 1;

FIG. 3 is a schematic diagram of another embodiment of a circuit foroperating the flood guns in the storage tube of FIG. 1 by automatictriggering;

FIG. 4 shows the voltage Waveforms applied to the cathode and controlgrid of the flood gun of FIG. 3; and

FIGS. 5A, 5B and 5C are diagrams of the waveforms of different types ofwriting speed enhancement voltage pulses which may be applied to theflood gun cathodes of FIG. 1 and the potentials of written andbackground target areas effected by such enhancement pulses.

One embodiment of the charge image storage apparatus of the presentinvention is shown in FIG. 1, and includes a conventional bistablestorage tube 10 or a direct viewing bistable storage tube similar tothat described in the copending U.S. patent application Ser. No.180,457, referred to above, so that such tube will not be discussed indetail. The electrical input signals to be displayed are applied acrossa pair of vertical deflection plates 12 at least one of which isconnected to an input terminal 14 through a vertical amplifier 16 and atwo position selector switch 18. The movable contact of the selectorswitch 18 is moved to the WRITE position shown, to apply the inputsignal to the vertical deflection plates during the writing operation ofthe storage tube. A pair of horizontal deflection plates 20 are alsoprovided within the storage tube and are connected to a horizontal sweepgenerator 22 through a second selector switch 24 whose movable contactis ganged to that of switch 18. Thus, the horizontal sweep generator 22applies a conventional saw tooth or ramp shaped sweep signal across thehorizontal deflection plates 20 when the selector switch 24 is in theWRITE position shown. As a result, a storage target 26 at one end of thetube 10 is bombarded by a narrow beam of high velocity writing electronswhich are emitted from a cathode 28 at the other end of such tube. Thewriting beam is deflected by the signals on the horizontal and verticaldeflection plates so that it produces a charge image on the storagedielectric of such target which corresponds to the waveform of thevertical input signal applied to input terminal 14.

If the current density of the writing beam emitted by cathode 28 issufficient, the voltage difference between such cathode and the target26 is high enough, and the speed of the horizontal sweep signal appliedis slow enough, the potential of the charge image produced on thestorage target 26 will be sufficient to enable bistable storage of suchcharge image for an indefinite controlable time. This bistable storageis caused in a conventional manner by bombarding such storage targetsubstantially uniformly with low velocity flood electrons emitted by apair of flood guns 30. Thus, when the potential of the charge imageproduced by the writing beam on the storage dielectric of target 26exceeds the first cross over voltage of the secondary emissioncharacteristic curve of such storage dielectric, the holding or floodelectrons drive the potential of the charge image up to a stable voltagenear the voltage of the collector electrode of such storage target. Atthe same time the unwritten background areas of the storage target whosepotential is below the first cross over voltage are driven downward to astable voltage adjacent the voltage of the cathodes 31 of the floodguns. In this manner, all areas of the rear surface of the storagetarget are held at one of these two stable voltages. It has beendiscovered that the effective first cross over voltage varies with thefield produced across the storage dielectric, at least for the phosphortarget referred to below, so that this term will be used to indicate theminimum charge voltage necessary for storage and may vary in value withthe voltage applied to the target electrode.

The storage target 26 may be similar to the direct viewing targetdisclosed in copending U.S. patent application Ser. No. 180,457,referred to above, or the split screen target shown in copending U.S.patent application Ser. No. 214,877, filed on Aug. 6, 1962, by Robert H.Anderson, entitled Storage Tube, now Patent No. 3,214,631. In eithercase, the storage dielectric is a thin layer of phosphor material whichserves the dual functions of storing the charge image bistably and ofconverting the charge image into a light image for direct viewing. Thisphosphor storage dielectric is supported over a light transparentelectrical conductive film of tin oxide coated on the rear surface ofthe face plate of the tube envelope. This conductive film serves as thecollector electrode for the secondary electrons emitted by the phosphorlayer due to the porous structure of such layer and is connected to atarget voltage produced across a fixed load resistor 32. The loadresistor 32 is connected in series with a variable resistor 33 be- Itween a source of positive D.C. supply voltage of about age above whichthe storage target is driven to a uniformly positive or completelyWritten condition by the flood electrons.

The low velocity holding or flood electrons emitted from the cathodes 31are normally transmitted through control grid 34 and anode 36 of theflood guns onto the surface of the storage target 26 after passingthrough at least one wall band electrode 38 of silver or otherconductive material coated on the inner surface of the funnel portion ofthe envelope. The wall band electrode 38 is connected to a positive DCbias voltage of about +50 volts when the flood gun cathode 31 isnormally grounded, in order to spread the flood electrons substantiallyuniformly over the surface of the storage target and to collimate suchflood electrons so that they strike the target at substantially rightangles thereto. It should be understood that while the Wall bandelectrode 38 is shown as a single electrode it may be provided as aplurality of spaced wall bands of varying potential for more precisecontrol of the flood electrons. Thus, while the writing beam of highvelocity electrons is focused into a narrow beam by transmitting itthrough a control grid 40 and a three element focusing anode structure42 in the writing gun so that the writing beam strikes the storagetarget as a small circular spot, the flood electrons are focused tocover the entire surface of the storage target. However, this flood guntype storage tube may be replaced by one in which the holding electronsare provided in the form of a narrow beam similar to that of the writingbeam but of lower velocity, which is deflected over the surface of thestorage target in a TV raster pattern to enable storage of the chargeimage in a similar manner to the flood electrons.

When an input signal of extremely high frequency or fast rise time isapplied to the vertical deflection plates 12 the potential of the chargeimage of such input signal produced on the storage target is sometimesbelow the first cross over voltage so that the charge image is notstored normally because the flood electrons drive its potential downtoward the voltage of the flood gun cathode along with the potential ofthe unwritten background target areas. It has been discovered that ifthe flood or holding electrons are prevented from bombarding the storagetarget during the time the writing electrons are producing the chargeimage thereon, an increase in the potential of such charge image resultswhich is frequently suflicient to enable bistable storage of such chargeimage when the holding electrons are subsequently allowed to bombard thestorage target. Furthermore, if the input signal is a repetitive signalit is possible to increase the potential of the charge image even moreby maintaining the flood guns turned off during several successivecycles of the input signal so that charge image of these successiveinput signals are superimposed on the storage target and theirpotentials effectively added together to increase the total potential ofthe resulting charge image over the first cross over voltage. Thischarge image integration technique is described in greater detail incopending US. patent application Ser. No. 302,880, by Robert H.Anderson, filed Aug. 19, 1963, entitled Improved Storage Tube and Methodof Operation.

The increase in the potential of the charge image achieved by preventingthe holding or flood electrons from striking the storage target duringwriting is due to the fact that such holding electrons oppose thewriting operation since they tend to drive the potential of the chargeimage downward to the voltage of the flood gun cathode. This opposingaction continues until the potential of such charge image is increasedabove the first cross over voltage. After this the holding electrons aidwriting by driving the potential of such charge image upward to thevoltage of the collector electrode.

It has also been discovered that separate areas of the same storagetarget initially at different potentials are charged at different ratesby the holding electrons to higher voltages when the voltage on theflood gun cathodes is lowered below the first cross over voltage, asshown in FIGS. 5A, 5B and 50. Thus, target areas of higher initialvoltage which have been struck by the writing beam are charged at afaster rate than unwritten background target areas of lower initialvoltage so that the voltage difference between such target areasincreases with time. It is therefore possible to diflerentiate between acharge image of very low initial voltage and the background areas of thetarget because of their different charging rates and to store the chargeimage by decreasing the flood gun cathode voltage back to zero after thepotential of the charge image exceeds the first cross over voltage butwhile the potentials of the background target areas are still below suchfirst cross over voltage. This allows the charge images of highfrequency signals to be stored and is especially useful when storinghigh speed transient signals but may also be employed along with thecharge image integration to store repetitive signals more quickly.

The writing speed enhancement technique described above is accomplishedin FIG. 1 by means of a manual switch 44 having three differentpositions with its movable contact connected to the flood gun cathodes31. The fixed contact of switch 44 labeled WRITE is connected to asource of positive DC. bias voltage of about volts through an isolatingresistor 46 so that the flood guns are cut off when the switch 44 is inthe WRITE position due to the fact that the control grids 34 areconnected to negative DC bias voltages of about 20 volts and such floodgun cathodes are therefore reverse biased by about volts. This preventsthe flood electrons from bombarding the storage target 26 during thetime the writing beam emitted by cathode 28 is producing the chargeimage on the storage target. In the STORE position of the manual switch44 the flood gun cathodes are connected across a capacitor 48 having oneterminal connected to ground and its other terminal connected to asource of negative D.C. supply voltage of about 70 volts through a fixedresistor 50 and a variable resistor 52 connected in series. Thecapacitor 48 is initially charged to -70 volts bu current flowingthrough resistors 50 and 52 so that by moving the switch 44 to the STOREposition the voltage on the flood gun cathode changes initially to 70volts. This causes the flood guns to be rendered conducting since theircathodes are then forward biased by 50 volts. Thus, flood electrons aretransmitted to the storage target of a higher voltage initially.However, the capacitor 48 immediately begins discharging toward thevoltage set at the common connection of such capacitor with resistor 50to decrease the velocity of the flood electrons exponentially. Thevoltage of the flood gun cathode 31 after discharge of capacitor 48 maybe set to 0 by adjusting the setting of resistor 52 to vary the currentso that the voltage drop across resistors 50 and 52 is 70 volts. As thevoltage on the flood gun cathodes is maintained at about 0 volts withrespect to ground, the storage tube 10 operates in a conventional mannerto store the charge image.

An electrical readout signal may be produced on the target coatingelectrode by scanning the phosphor layer of the storage target with areading beam of electrons as described in copending US. patentapplication Ser. No. 245,716, filed Dec. 19, 1962, by Robert H. Andersonand entitled Electrical Readout for Storage Tube, now Patent No.3,219,316. This reading beam may be produced by the same electron gunwhich provided the writing beam merely by changing the position of theswitches 18 and 24 to the READ position indicated so that a rastersignal generator 54 is connected to the horizontal and verticaldeflection plates of tube 10 to move the beam in a TV raster patternover the surface of the target. In order to prevent this reading beamfrom storing the raster pattern, it is necessary either to connect thecathode 28 to a more positive voltage to decrease the velocity of thereading electrons below that of the writing electrons, or to increasethe negative voltage applied to the control grid 40 in order to decreasethe current density of the reading beam. This latter technique is moredesirable and may be 7 accomplished by means of a switch 56 whosemovable contact is connected to the control grid 40 so that in the WRITEposition of switch 56 the control grid is connected to a negative DC.voltage source of about 3,025 volts, while in the READ position of suchswitch the control grid is connected to a voltage of about -3,050 volts.The electrical readout signal produced on the conductive film of thestorage target is transmitted through a coupling capacitor 58 to a lowinput impedance preamplifier 60. The output of preamplifier 60 isconnected through a conventional voltage amplifier 62 before beingtransmitted to the Z-axis input at the control grid or cathode of aremote TV monitor tube 64. The horizontal and vertical deflection platesof the monitor tube 64 are also connected to the raster signal generator54 so that the same or related saw tooth raster signals may be appliedto these deflection plates as are applied to the horizontal and verticaldeflection plates of the storage tube during the readout operation. As aresult, the wave form image stored on the storage target 26 isreproduced on the fluorescent screen of the monitor tube 64. Of course,electrical readout is not necessary when a direct viewing storage targetis employed but the present method may also be employed with storagetubes having nondirect readout targets.

The wave form image stored on the target 26 may be removed by aconventional erase operation merely by varying the resistor 33 so thatthe voltage across load resistor 32 is first increased above the fadepositive voltage to enable the flood electrons to cause the storagetarget to fade uniformly positive. Then the target voltage is decreasedbelow the first cross over or retention threshold voltage to cause thepotential of the rear surface of the storage dielectric to be drivennegative back to the voltage of the flood gun cathode. Next, the voltageacross resistor 32 is slowly increased above the first cross overvoltage so that the conductive film target electrode is provided with avoltage within the stage range without causing the rear surface of thestorage dielectric to follow such target electrode voltage. This eraseoperation may also be accomplished by pulsing the target electrode.Also, while it is not essential, it has been found desirable to connectthe flood gun cathode to a source of positive DC. bias voltage of, forexample, about +50 volts during the erase operation. This may beaccomplished by rotating the movable contact of switch 44 to the ERASEposition.

The potential of the flood gun cathode during the operation of the tubeof FIG. 1 is shown by the curve 66 in FIG. 2. It should be noted thatthe writing rate enhancement pluse portion 68 is applied immediatelyafter the WRITE operation during which the flood gun cathode is cut off.This enhancement pulse 68 decreases to a negative voltage which may besubstantially below the first cross over voltage and then rises in anexponential manner towards volts at a rate determined by the RC. timeconstant of the circuit including capacitor 48 and resistors 50 and 52.Thus, the width of such enhancement pulse is approximately equal to 3 RCand should be inversely proportional to the voltage amplitude of suchenhancement pulse. A large voltage writing rate enhancement pulse willcause the background areas of the target to charge more rapidly in apositive direction due to the increased secondary emission caused by thegreater velocity of the flood electrons so that the enhancement pulsemust be terminated sooner to prevent the voltage of such backgroundareas from exceeding the first cross over voltage. In order to providean enhancemeent pulse of the correct width, the proper values ofresistance and capacitors must be selected for resistors 50 and 52 andcapacitor 48.

Another embodiment of the method and apparatus for increasing thewriting rate of the storage tube of FIG. 1, is shown in FIG. 3. Theflood guns 30 are automatically turned off at the start of the verticalinput signal applied through the vertical amplifier 16 to the verticaldeflection plates of the storage tubes. This may be accomplished bytransmitting a portion of the vertical input signal through a sweeptrigger generator 69 to produce trigger pulses at the start of suchvertical signal which are transmitted to the horizontal sweep generator22 to start the operation of such sweep generator in a conventionalmanner. The sweep trigger generator 69 may also be connected to theinput of a flood gun blanking multivibrator 70 to trigger suchmultivibrator so that it produces a negative voltage output pulse 72.This negative output pulse is then transmit-ted to the control grid 34of the flood gun and functions as a blanking pulse to reverse bias thecathode of such flood gun to cut 01?. This allows the capacitor 48 tocharge by current flowing through resistors 50 and 52 from the 70 voltDC. voltage source until voltage across such capacitor reaches that ofthe voltage source. After the charge image of the vertical input signalis written on the storage target, the blanking pulse 72 is terminated toreturn the control grid 34 to a more positive voltage so that the floodgun is again rendered conducting. At this time the voltage on the floodgun cathode 31 is equal to the 70 volts of the fully charged capacitor48. However, thi cathode voltage decreases as the capacitor dischargesto a more positive voltage which may be set to zero by varying resistor52 until the beam current produces a voltage drop of 70 volts acrossresistor 50 and 52.

The flood gun blanking pulse generator 70 may be a monostablemultivibrator whose frequency is separately controlled so that it isconsiderably lower than the frequency of the input signal applied toinput terminal 14 in order to enable charge image integration bymaintaining the flood gun cut off during several successive cycles ofsuch vertical input signal. However, it may also be desirable to connectthe flood gun blanking pulse generator 70 as a bistable multivibratorwhich is triggered by sweep trigger pulses and is reverted to itsinitial stable state by a signal from the horizontal sweep generatorcorresponding to the retrace portion of the horizontal sweep signal sothat the blanking pulse 72 is discontinued immediately after thehorizontal sweep signal. This bistable flood gun blanking multivibratormay be desirable when operating the storage tube to store transientvertical input signals. It should be noted that a different type ofsignal generator may be employed in place of the blanking multivibrator70 and the output of this signal generator may be connected to the floodgun cathode 31 to apply a positive voltage blanking pulse as well as awrit ing speed enhancement pulse to such cathode without employing theseparate pulse forming circuit of resistors 50 and 52 and capacitor 48.

It is extremely desirable to provide the triggered writing rateenhancement operation of FIG. 3 when attempting to store a high speedtransient input signal because the charge image produced by suchtransient signal on the storage target could decrease below the minimumvoltage level necessary for storage due to leakage if an enhancementpulse is not applied to turn on the flood gun im mediately after thecharge image is produced. For this reason a bistable multivibrator ismore desirable as the flood gun blanking multivibrator.

The flood gun blanking pulse 72 produced by the apparatus of FIG. 3, isshown in time relation to the flood gun cathode voltage 74 in FIG. 4.Thus, when the blanking pulse 72 is applied to the control grid 34, theflood gun cathode voltage gradually decreases from "0 to about -70 voltsas the capacitor 48 charges. When the blanking pulse 72 is terminated,the flood gun cathode voltage then increases from 70 volts to 0 volts asthe capacitor 48 discharges. This positive going portion of the floodgun cathode voltage 74 provides the writing speed enhancement pulse. Itshould be noted that the flood gun blanking multivibrator 70 of FIG. 3is provided with a lock out control which prevents such multivibratorfrom being retriggered during storage and until after the charge imagehas been erased from the storage target.

The effect of operating a storage target in accordance with the methodsof the present invention is shown by the curves of FIGS. A, 5B and 5C.When the writing speed enhancement pulse applied to the flood guncathode is in the form of a negative spike voltage 76 as shown in FIG.5A, the best results are obtained. This is due to the separation of thecharge image voltage 78 from the background voltage 80 of unwrittentarget areas by a greater amount Y when the charge image voltage exceedsthe first cross over voltage V Before the enhancement pulse 76 isapplied the charge image voltage 78 differs from the background voltage80 by a small amount X after such charge image is written on the storagetarget, but before the flood electrons strike such target. When theenhancement pulse 76 is applied to the flood gun cathode, the potentialof the charge image and the potential of the background target areasbegin to rise due to the charging action of the flood electrons. Asstated previously, the charge image voltage increases at a faster ratethan the background voltage due to the fact that it Was initially at ahigher potential. Therefore, at some time after the enhancement pulse isapplied, the charge image voltage 78 and the background voltage 80 areseparated by a greater voltage difference than X. This enables theenhancement pulse 76 to be terminated within a wider time range withoutcausing background to fade positive since the charge image voltage 78exceeds the first cross over voltage V much sooner than the backgroundvoltage. Of course, the writing speed enhancement pulse 76 must beterminated or decreased in voltage to such a value that it can no longerincrease the background voltage above the first cross over voltage,before such background voltage exceeds V in order to cause bistablestorage.

When the charge image voltage 78 is driven above the first cross overvoltage, the flood electrons cause such charge image voltage to bedriven upward to a high voltage stable state V which is slightly greaterthan the voltage applied to the collector electrode At the same timesuch flood electrons cause the background voltage 80 of the unwrittentarget areas charged to a potential below V to decrease in voltage to apotential approximately equal to thatof the flood gun cathode.

As shown in FIG. SE, a similar charging operation takes place when anegative rectangular writing rate enhancement pulse 82 is applied to theflood gun cathode, However, the increase in the voltage differencebetween the charge image voltage 78 and the background voltage 80' isnot as great. Thus, the voltage difference Y between curves 78' and 80when curve 78 exceeds V is less than the voltage difference Y in FIG. 5Aeven though the initial voltage difference X is the same in both cases.This means that the width W of the rectangular enhancement pulse 82 ismore critical than the width of the spike enhancement pulse 76, so thatsuch rectangular enhancement pulse must terminate immediately after thecharge image voltage 78' exceeds V Of course, the maximum amplitude V ofboth of the enhancement pulses 76 and 82 may also exceed the amplitudeof the first cross over voltage V because they are maintained above thatvoltage for only a short time.

In FIG. 5C a negative stair step enhancement pulse 84 is applied to theflood gun cathode. Since this stair step enhancement pulse is maintainedat its maximum voltage V and does not return to 0 its amplitude must beless than that of the first cross over voltage V Also, the sum of thevoltage V of the enhancement pulse and the initial voltage A of thecharge image voltage 7 8" must be greater than the first cross overvoltage, while the sum of V and the initial voltage A of the backgroundvoltage 80" must be less than the first cross over voltage. In otherwords, A must be greater and A less than the difference in the voltageAV between the stair step enhancement voltage V and the first cross overvoltage -V in order to enable bistable storage. Of course, thistechnique is less desirable than the other two discussed because thedifferential charging action of the methods of FIGS. 5A and 5B is notemployed. Thus, the initial voltage difference X between the chargeimage voltage and the background voltage must be sufficient to enablethe enhancement voltage 84 to cause the charge image voltage to exceedthe first cross over voltage immediately without causing the backgroundvoltage to exceed the first cross over voltage. This means that anyappreciable fiuctation in the voltage V of the enhancement pulse 84would prevent the bistable storage of charge images of very low initialpotential due to the fact that there is always some noise in thebackground voltage. It should be noted that this noise voltage is lessfor smooth surface storage targets such as the phosphor storage targetshown in copending US. patent application Ser. No. 180,457, describedabove, than it is for conventional direct viewing storage targets of thecurrent transmission type employing a mesh structure, due to the factthat the thickness of the phosphor target is substantially uniform whilethe thickness of the mesh target varies considerably.

Of course it is possible to use a rectangular or spike enhancement pulsein place of pulses 82 or 76 whose maximum negative voltage V is lessthan V if the potential difference AV between V and V is maintained withthe same relationship to A and A as in FIG. 5C. However the width of theenhancement pulse must be increased considerably with written areas oflow initial voltage in order to raise the potential of such areas aboveV As a result of this increased charging time the potential of theunwritten target areas may decrease along curve 80" of FIG. 5 C to anegative voltage which is greater than the potential difference betweenthe target electrode voltage V, and the fade positive voltage, beforethe enhancment pulse terminates. If this happens the electrical fieldacross the target dielectric collapses and the potential of thebombarded surface of the target increases to a uniformly positivevoltage equal to the voltage of the target electrode so that it has theeffect of erasing the charge image. When an enhancement pulse having amaximum negative voltage V greater than V is employed, this problem isavoided because the unwritten target areas charge positively alongcurves 80 and 81 of FIGS. 5A and 5B and decrease, rather than increase,the field across the target dielectric.

It will be obvious to those having ordinary skill in the art thatvarious changes may be made in the details of the above preferredembodiment of the present invention without departing from the spirit ofthe invention. For example, when an ion repeller electrode or asecondary electron collector electrode is employed between the storagetarget and the flood gun, it would be possible to apply the blankingpulse to such electrode in order to prevent the flood electrons frombombarding the storage target during writing. Also, the writing speedenhancement pulse may be applied to the storage target electrode insteadof the flood gun cathode merely by reversing the polarity of such pulse.Furthermore it is not essential that the flood guns be completely cutoff during writing to prevent all target areas from being bombarded byflood electrons at this time. On the contrary, it is only necessary toprevent the flood electrons from striking those areas of the storagedielectric at a potential less than the first cross over Voltage, duringwriting. Thus flood electrons may be allowed to strike target areascharged above such first cross over voltage at this time since they donot oppose writing in these areas. This may be accomplished byincreasing the potential of the flood gun cathodes with respect to therear surface of the target dielectric in the positive direction until itis slightly above the first cross over voltage. The potential of thecontrol grid of the flood gun is also increased so that flood electronsare still emitted therefrom. However most of these flood electrons arecollected by the Wall band electrodes 38 and no flood electrons strikethe storage target except in those written areas which the writing beamhas driven above the first cross over voltage. As in the other methodsdescribed above, the negative writing rate enhancement pulse is appliedto the flood gun cathodes after the writing operation to enable floodelectrons to strike all target areas. Therefore, the scope of thepresent invention should only be determined by the following claims.

We claim: 1. A method of increasing the writing rate of a bistablestorage tube comprising the steps of:

forming a charge image on the storage dielectric of a storage target insaid tube with at least a portion of said charge image having apotential less than the first crossover voltage of the secondaryemission characteristic of said storage dielectric which is the minimumvoltage necessary for bistable storage;

bombarding said storage dielectric with low velocity holding electronsto raise the potential of said portion of said charge image above saidfirst crossover voltage without increasing the potential of theunwritten background areas of the storage dielectric above said firstcrossover voltage in order to cause bistable storage of the entirecharge image;

applying an enhancement voltage to said tube during said bombardment bysaid holding electrons to produce a positive voltage difference betweenthe cathode emitting the holding electrons and the storage dielectricwhich is greater than the first crossover voltage, thereby increasingthe velocity of said holding electrons for a brief time at the beginningof bombardment of said storage dielectric by said holding electrons toincrease the potential of said portion of said charge image above saidfirst crossover voltage; and

terminating the enhancement voltage to reduce said voltage differencebelow said first crossover voltage thereby decreasing the velocity ofsaid holding electrons to a lower velocity in time to prevent thepotential of said background areas from exceeding said first crossovervoltage while continuing to bombard said storage dielectric with saidholding electrons to store said charge image.

2. A method in accordance with claim 1 in which the charge image isformed by bombarding the storage dielectric with high velocity writingelectrons.

3. A method in accordance with claim 2 in which the low velocity holdingelectrons are prevented from striking the storage dielectric duringbombardment of said storage dielectric by the Writing electrons to formthe charge image.

4. A method in accordance with claim 3 in which a substantially constantvoltage is applied to a target electrode over which the storagedielectric is supported, during the time the holding electrons areprevented from striking the storage dielectric.

5. A method in accordance with claim 3 in which a repetitive inputsignal is applied to the tube to modulate the writing electrons and forma charge image of the input signal, and several successive cycles ofsaid input signal are applied during the time the holding electrons areprevented from striking the storage dielectric to enable integration ofthe superimposed charge images of said succcessive cycles.

6. A method in accordance with claim 1 in which the enhancement voltageis a spike shaped pulse whose trailing edge decreases at a slower ratethan its leading edge increases.

7. A method in accordance with claim 1 in which the enhancement voltageis a rectangular pulse.

8. Electron image storage apparatus, comprising:

a storage target including a secondary emissive storage dielectriccapable of bistable storage;

means for bombarding said storage dielectric with high velocity writingelectrons to produce a charge image on said storage dielectric, at leasta portion of said charge image having a potential less than the firstcrossover voltage of the secondary emission characteristic of saidstorage dielectric;

means including a holding cathode for bombarding said storage dielectricwith low velocity holding electrons so that said holding electrons causebistable storage of said charge image when its potential is above saidfirst crossover voltage;

means for applying an enhancement voltage to said tube to produce apositive voltage difference between the holding cathode and the storagedielectric which is greater than the first crossover voltage, therebyincreasing the energy of said holding electrons for a brief time at thebeginning of their bombardment of said storage dielectric to increasethe potential of said portion of said charge image above said firstcrossover voltage; and

means for terminating said enhancement voltage to reduce said voltagedifference below the first crossover voltage, thereby decreasing theenergy of said holding electrons to a lower energy while continuing tobombard said storage dielectric with said holding electrons, in time toprevent the potential of the unwritten background areas of said storagedielectric from exceeding said first crossover voltage.

9. An apparatus in accordance with claim 8 in which the storage targetis contained within the evacuated envelope of a storage tube formingpart of said apparatus.

10. An apparatus in accordance with claim 9 which also includes meansfor preventing the holding electrons from striking the storagedielectric during the bombardment of said storage dielectric by saidwriting electrons to form the charge image.

11. An apparatus in accordance with claim 10 which also includes meansfor applying a repetitive input signal to the storage tube to modulatethe writing electrons and form a charge image of the input signal, andfor applying several successive cycles of said input signal to said tubeduring the time the holding electrons are prevented from bombarding thestorage dielectric to enable integration of the superimposed chargeimages of said successive cycles.

12. An apparatus in accordance with claim 10 which also includes meansfor applying a substantially constant voltage to a target electrode overwhich the storage dielectric is supported.

13. An apparatus in accordance with claim 10 which includes a switchmeans for applying a negative bias voltage between the holding cathodeand its control grid to cut off the flow of holding electrons to thestorage dielectric, and for applying the enhancement voltage to theholding cathode.

14. An apparatus in accordance with claim 9 in which the storagedielectric is a layer of phosphor material provided over a lighttransparent conductive film coated on the inner surface of the faceplate of the storage tube.

References Cited UNITED STATES PATENTS 3,259,791 7/1966 Jensen et al315-12 FOREIGN PATENTS 11,519 7/1961 Japan.

RICHARD A. FARLY, Primary Examiner.

C. L. WHITHAM, Assistant Examiner.

